Shape Memory Alloy Single Crystals Research Articles

Austenite-martensite interfacial patterns and energy dissipation of phase transformation in Ni-Mn-Ga single crystal

The martensitic phase transformation occurs in Shape Memory Alloys (SMA) via the nucleation/propagation of the Austenite-Martensite interface (A-M interface), which is a transition region (domain) between the two coexisting phases. For providing compatibility, the transition region consists of various martensitic twin structures (laminates), forming different interfacial patterns, such as parallel laminates and branching laminates. Due to the energy accumulation in the interfacial structures (e.g., elastic mismatch and twin-boundary surface energy), the interfacial patterns should be relevant to the driving force (energy dissipation) and the associated kinetics of the phase transformation. In this paper, we adopt a special thermal loading, small-temperature-gradient “heating-cooling-reheating”, to control the A-M interface's forward and reverse propagation to generate different interfacial patterns in a Ni-Mn-Ga single crystal SMA with the observation on the twin structures (by optical microscope and SEM) and the InfraRed measurement on the temperature hysteresis of the interface propagation (for characterizing the thermal driving force). Simple energetic analysis indicates that the thermal driving force (energy dissipation) is directly related to the stored energy in the interfacial structure, particularly the large mismatch elastic energy near the habit plane. This study not only provides the details of the various interfacial patterns and their dependence on the loading path, but also indicates the driving force and the associated mechanism about the pattern evolution to understand the phase transformation process.

Effects of machining parameters on Ni-Mn-Ga-based alloys for fabrication of multifunctional micro devices using femtosecond pulse width laser

Ni-Mn-Ga-based magnetic shape memory (MSM) alloy single crystals are known for their large magnetic-field-induced strain (MFIS). This quality makes them a promising material for use in micro actuators and devices. However, the manufacturing of single-crystal-based Ni-Mn-Ga micro actuators is challenging due to their high brittleness and other material properties – numerous machining techniques that are successfully used for the deep engraving of conventional engineering materials cannot be directly applied to Ni-Mn-Ga-based alloys. Nevertheless, previous studies have shown that a femtosecond pulse width laser (FPWL) can be successfully utilized for the defect-free micromachining various materials. This work studies the effects of different engraving parameters and introduces a novel scanning-based method for the deep micromachining of Ni-Mn-Ga-based MSM alloys with maximum surface quality. Results show that a 4-layer strategy with a 0.01 mm hatch distance provides excellent machining in terms of surface quality and dimensional accuracy. This study can be utilized within design stages to estimate minimum margin based on required machined depth and avoid defects that occur in the sample preparation stage. Additionally, evolution of structures generated by FPWL machining are characterized. The results highlight how FPWL can be considered a highly capable process for the micromachining and surface structuring of Ni-Mn-Ga-based single crystals for manufacturing multifunctional MSM microdevices.

Fabrication of single-crystal Fe-Mn-Al-Cu alloys by cyclic heat treatments

In general, single crystal shape memory alloys exhibit superior shape memory effect and superelasticity when compared to their polycrystalline counterparts. However, it is challenging and costly to prepare a single crystal in traditional ways, such as the Czochralski method and directional solidification technique. Recently, cyclic heat treatment (CHT) has emerged as a facile and cost-effective method to fabricate single crystal by inducing abnormal grain growth. In the present work, we aimed to fabricate Fe-Mn-Al-Cu single crystals utilizing the CHT technique. Single crystal Fe-Mn-Al-Cu shape memory alloys were successfully prepared, and the effect of CHT parameters on abnormal grain growth was investigated. It was revealed that increasing the cooling rate resulted in a non-monotonic response in the maximum grain size, which initially increased and was followed by a subsequent decrease. This behavior was attributed to the subgrain refinement, increase in subgrain misorientation angles, and the narrowing of the low density zones (LDZs) of subgrains. Decreasing the lower temperature of CHT caused similar trends in maximum grain size, due to the increase in subgrain misorientation angle and the expansion of the LDZs. Extending the holding time at the lower temperature of CHT had minimal impact on grain size. Increasing the heating rate of CHT resulted in a reduction in the maximum grain size because of the narrowing of the LDZs. Moreover, the addition of 2 wt% Cu led to a reduction of the maximum grain size, attributing to the decreased solvus temperature of the γ phase as a result of Cu addition. Consequently, the γ phase and subgrains were refined, and the LDZs were narrowed.

The effect of microstructural barriers on transient crack growth in shape memory alloys

There are several issues to be solved in the fracture mechanics of shape memory alloys, one of them being the resistance to crack growth and therefore to fracture. This paper discusses the crack growth in a single crystal CoNiAl shape memory alloy under cyclic loading and the effect of micro-structural barriers. To observe the crack growth in detail, tests are conducted on edge-notched specimens. The displacement field is obtained using digital image correlation (DIC), and the fracture parameters are calculated by fitting anisotropic crack tip displacement equations to DIC data. Similar crack growth behaviors are observed in both superelastic and shape memory specimens, with a comparatively higher crack growth rate in the superelastic case: first a crack initiates at the notch and grows, then new cracks are observed to form near the tip of the main crack, or on the notch when the growth slows down. Then, further cyclic loading leads to the growth of the main crack and the new crack simultaneously with the two cracks merging at the end. Test specimens are examined post-failure with optical microscopy to better understand this complicated behavior. Results showed the presence of a non-transforming secondary (γ) phase around the regions where the propagating cracks slowed down, deviated, and/or stopped, improving the resistance of the shape memory alloy specimen to fracture.

Surface Effect on Phase Transformation of Single Crystal NiTi Shape Memory Alloys Studied by Molecular Dynamics Simulation

Surface effect on phase transformation of single crystal (SC) NiTi shape memory alloys (SMAs) with various thicknesses and various Ni contents is studied by molecular dynamics simulation. For the SMAs with various thicknesses considering surface effect, the residual strain, the average atomic potential energy, and the four characteristic phase transformation temperatures all increase, and fewer twinned martensite boundaries are left after cooling compared to the sample without considering surface effect. Moreover, with increasing thickness of the sample, the surface effect is gradually weakened, and there is no obvious regularity in phase transformation characteristics. For the SMAs with various Ni contents considering surface effect, segmental martensite phase transformation, phase transformation fluctuation, and wider phase transformation temperature ranges are observed, whereas these phenomena do not occur in the sample without considering surface effect. With the increasing of Ni contents, for both cases of considering surface effect or not, the critical stress of phase transformation, average atomic potential energy, the modulus of detwinned martensite increase, but the phase transformation temperature and modulus of twinned martensite decrease.

Crossover from mechanical stabilization of martensite to spanning avalanche-type reverse transformation of deformed Cu-Al-Ni shape memory crystals

A new interpretation of factors controlling the kinetics of reverse transformation of martensites stabilized by prestrain is suggested. We investigate the effect of amount of prestrain in the β1´martensite and in the β-phase on temperature and kinetics of the reverse martensitic transformation in Cu-Al-Ni shape memory alloy single crystals. Calorimetry and video recording of the jumping samples, provoked by the burst strain recovery during temperature-induced reverse transformation, are used in experiments. For crystals deformed both in the β´1 martensite and in the β-phase, we report the existence of a sharp transition (crossover) from the mechanical stabilization of martensite to the burst reverse martensitic transformation which proceeds as a single avalanche. The crossover occurs for prestrain values close to the maximum transformation strain. The specific energy of the jump during burst strain recovery depends on the maximum stress applied during prestraining the sample. The crossover from a broad transformation range to an infinite or spanning avalanche is predicted by 3D random field Ising model for disordered systems undergoing the first order phase transition. Based on this model, we explain the crossover in deformed Cu-Al-Ni crystals by a rapid decrease of structural disorder towards the critical one at the final stage of the formation of detwinned γ´1martensite. A simple solution is obtained for the critical transformation rate which produces jumping of the sample during burst reverse transformation after prestrain.

Hybrid Output Regulation of Hysteretic Actuators based on Single-Crystal Shape Memory Alloy Wires

In this paper, we present a novel hybrid feedback controller for a hysteretic actuator based on smart materials. The system under investigation consists of a single-crystal Shape Memory Alloy wire coupled with a biasing mechanism. Starting from a physics-based hybrid description of the nonlinear actuator behavior, a Throw-Catch hybrid strategy is developed to achieve asymptotic regulation of the system output (i.e., position) based on a combination of global and local control schemes. The control law permits the desired regulation goal to be achieved without the need to measure or estimate the complete system state, by solely relying on the actuator output information. After being described, the controller is validated via simulations.

Hysteresis effect on austenite-martensite interface in Ni-Mn-Ga single crystal

The experiments on Ni-Mn-Ga single crystal shape memory alloy under a heating-cooling cycle demonstrate that the automatically generated twin laminate structure of the compatible austenite-martensite interface in the forward martensitic phase transition is significantly different from that in the reverse phase transition, even though the temperature hysteresis is small (As−Ms around 4 °C). Moreover, after the cooling-induced austenite → martensite transition, the remaining twin laminate is so fine (layer width around 1 μm) that the neighbouring twin boundaries merge with each other and disappear, making the fine twin laminate evolve into a single martensite variant (i.e., spontaneous detwinning). These observations provide insights into the relation between the hysteresis (phase-transition driving forces), austenite-martensite interface, and the basic material parameters such as twin boundary energy and softening modulus during the phase transition.

Atomistic simulations of AuTi high-temperature shape memory alloys

Herein, the atomic-scale phase transformation and deformation processes of AuTi shape memory alloys for high-temperature applications were investigated using molecular dynamics simulations based on a newly developed interatomic potential for the Au-Ti binary system. The developed second nearest-neighbor modified embedded-atom method potential exhibited good accuracy and transferability to reproduce various physical properties of the target alloy system, particularly in properties related to the reversible phase transformation of AuTi high-temperature shape memory alloys. As a conceivable application of the interatomic potential, the temperature-induced phase transformation of AuTi single-crystal shape memory alloys and the effect of off-stoichiometry on the phase transformation behavior were examined. Molecular dynamics simulations revealed that the transformation temperature and thermal hysteresis decreased with increasing Ti content, consistent with the reported experimental trend, suggesting that the reduced volume difference between the austenite and martensite phases was closely related to the reduced thermal hysteresis. We further examined the mechanical response and resultant superelasticity of nanocrystalline AuTi alloys to demonstrate that the cyclic deformation of nanocrystalline alloys exhibited a transformation ratcheting behavior caused by the plastic deformation of amorphous-like grain boundary region.

Phase field simulation on the martensite transformation and reorientation toughening behaviors of single crystal NiTi shape memory alloy: Effects of crystalline orientation and temperature

Based on the Ginzburg-Landau’s theory, a two-dimensional phase field model coupling martensite transformation/reorientation (MT/MR) and crack behavior was established. The crack propagation of single crystal NiTi shape memory alloy (SMA) and its dependence on the system’s crystalline orientation and temperature was studied by the phase filed simulations. The simulated results show that the proposed phase field model reasonably characterizes the crack propagation, MT/MR and stress–strain curve of single crystal NiTi SMA with different crystalline orientations and at various temperatures. The MT/MR toughening behaviors occur in the simulations, which depend greatly on the crystalline orientation and temperature: when the temperature is higher than the chemical equilibrium temperature, the MT toughening effect decreases with the increase of temperature; when the temperature is set to be lower than the chemical equilibrium temperature, the MR toughening effect decreases with the decrease of temperature. Generally, the closer the temperature to the chemical equilibrium temperature, the better the MT toughening effect and the weaker the orientation dependence is. However, the change of MR toughening capability with the temperature and crystalline orientation is complex because the twinned martensite phase at the crack tip can not only slow down the crack propagation rate, but also induce the crack bifurcation.

Quantitative reorientation behaviors of macro‐twin interfaces in shape‐memory alloy under compression stimulus in situ TEM

Twinning stress is known to be a critical factor for the actuating performance of magnetic shape memory alloys because of the harmful deterioration of their magnetic field-induced strain effect. However, the intrinsic origin of the high twinning stress is still in debate. In this work, we firstly fill this gap by precisely probing the reorientation behaviors of A-C and A-B two common macro-twin interfaces under the stimulus of uniaxial compression in-situ transmission electron microscope. The grain boundary is proved to be the main reason for large twinning stress. The twinning stress of the A-C and A-B type interfaces quantitatively are ∼0.69 and 1.27 MPa within the plate respectively. The A-C type interface evidently has smaller twinning stress and larger deformation variable than the A-B interface. Under the action of compression, not only the orientations of the crystals have changed, but also the roles of the major and minor lamellae have changed for both interfaces due to the movements of twinning dislocations. Combining in-situ and quasi in-situ electron diffraction data, the reorientation process is clearly and intuitively shown by the stereographic projection. Atomic models and the theory of dislocation motion are proposed to phenomenologically clarify the intrinsic mechanism. This work is believed to not only provide a deeper understanding of the deformation mechanism of magnetic shape memory alloys under uniaxial compression testing, but also discover that compression training is not the mechanical training way to decrease the twinning stress of non-modulated martensite in single crystal shape memory alloys.

On the impact of nanometric γ’ precipitates on the tensile deformation of superelastic Co49Ni21Ga30

Results are presented reporting on the martensite domain variant selection and stress-induced martensite morphology in [001]-oriented superelastic Co49Ni21Ga30 shape memory alloy (SMA) single crystals under tensile load. In situ neutron diffraction, as well as in situ optical- and confocal laser scanning microscopy were conducted focusing on three differently treated samples, i.e. in the as-grown, solution-annealed and aged condition. An aging treatment performed at 350 °C promotes the precipitation of nanoprecipitates. These second phase precipitates contribute to an increase of the number of habit plane interfaces, while reducing lamellar martensite plate thickness compared to the as-grown and solution-annealed (precipitate free) samples. During tensile loading, all samples show a stress-induced formation of martensite, characterized by one single domain variant (“detwinned”) and one set of parallel habit planes in a shear band. The results clearly show that γ’ nanoprecipitates do not necessarily promote multi-variant interaction during tensile loading. Thus, reduced recoverability in Co-Ni-Ga SMAs upon aging cannot be solely attributed to this kind of interaction as has been proposed in literature so far.

Effects of aging on the stress-induced martensitic transformation and cyclic superelastic properties in Co-Ni-Ga shape memory alloy single crystals under compression

Co-Ni-Ga shape memory alloys attracted scientific attention as promising candidate materials for damping applications at elevated temperatures, owing to excellent superelastic properties featuring a fully reversible stress-strain response up to temperatures as high as 500 °C. In the present work, the effect of aging treatments conducted in a wide range of aging temperatures and times, i.e. at 300–400 °C for 0.25–8.5 h, was investigated. It is shown that critical features of the martensitic transformation are strongly affected by the heat treatments. In particular, the formation of densely dispersed γ’-nanoparticles has a strong influence on the martensite variant selection and the morphology of martensite during stress-induced martensitic transformation. Relatively large, elongated particles promote irreversibility. In contrast, small spheroidal particles are associated with excellent functional stability during cyclic compression loading of 〈001〉-oriented single crystals. In addition to mechanical experiments, a detailed microstructural analysis was performed using in situ optical microscopy and neutron diffraction. Fundamental differences in microstructural evolution between various material states are documented and the relations between thermal treatment, microstructure and functional properties are explored and rationalized.

Spontaneous Martensite Detwinning in Ni-Mn-Ga Single Crystal

It is well known that the transformation between cubic Austenite phase and tetragonal Martensite phase of Ni-Mn-Ga single crystal Shape Memory Alloy can be controlled by thermo-magneto-mechanical fields. Particularly, the directional driving forces (magnetic fields and mechanical forces) can also trigger Martensite detwinning or Martensite variant reorientation due to the anisotropy of the tetragonal Martensite variant. By contrast, the temperature (a scalar quantity) was not expected to trigger Martensite detwinning. However, our experiments show that the cooling-induced Austenite → Martensite transformation is via two steps: Austenite → twin (consisting of two different Martensite variants), and twin → single-Martensite-variant via a detwinning process. The result implies that the material can switch between the Austenite phase and a single Martensite variant with a large cyclic macroscopic deformation in the simple heating-cooling cycle (a pure thermal loading without external stress or magnetic field), so-called stress-free “Two-Way Memory”.

A Hybrid Dynamical Modeling Framework for Shape Memory Alloy Wire Actuated Structures

In this letter, a hybrid model for single-crystal Shape Memory Alloy (SMA) wire actuators is presented. The result is based on a mathematical reformulation of the Müller-Achenbach-Seelecke (MAS) model, which provides an accurate and interconnection-oriented description of the SMA hysteretic response. The strong nonlinearity and high numerical stiffness of the MAS model, however hinder its practical use for simulation and control of complex SMA-driven systems. The main idea behind the hybrid reformulation is based on dividing the mechanical hysteresis of the SMA into five operating modes, each one representing a different physical state of the material. By properly deriving the switching conditions among those modes in a physically-consistent way, the MAS model is effectively reformulated within a hybrid dynamical setting. The main advantage of the hybrid reformulation is the possibility of describing the material dynamics with a simplified set of state equations while maintaining all benefits of the physics-based description offered by the MAS model. After describing the novel approach, simulation studies are conducted on a flexible robotic module actuated by protagonist-antagonist SMA wires. Through comparative numerical analysis, it is shown how the hybrid model provides the same accuracy as the MAS model while saving up to 80% of the simulation time. Moreover, the new modeling framework opens up the possibility of addressing SMA control from a hybrid systems perspective.

Impact of test temperature on functional degradation in Fe-Ni-Co-Al-Ta shape memory alloy single crystals

The present paper focuses on the analysis of functional fatigue properties in 〈001〉-oriented single crystalline Fe-Ni-Co-Al-Ta shape memory alloys. Superelastic cycling experiments up to 4.5% at different temperatures were conducted and revealed excellent cyclic stability at lower testing temperatures. Transmission electron microscopy observations shed light on the influence of precipitation and dislocation activity on functional stability.

Finite element and experimental structural analysis of endodontic rotary file made of Cu-based single crystal SMA considering a micromechanical behavior model

Shape Memory Alloys (SMAs) are widely used in endodontics as instruments for root canal preparation namely endodontic rotary files. Despite their high performances, compared to stainless steel instruments, it is still possible to enhance their cutting efficiency by acting on their shape and material properties. As an example, increasing the exhibited reversible martensitic transformation strain makes them more flexible without decreasing their mechanical strength. It becomes possible with single crystal SMAs. Cu-based (Cu-Al-Be, Cu-Zn-Al, Cu-Al-Mn) single crystal SMAs start emerging and can reach about 12% of martensitic transformation strain, in addition to their interesting antimicrobial properties. This study investigates, both numerically and experimentally, the development of endodontic files made of Cu-based single crystal SMAs. The numerical analysis is carried out by the finite element method. The geometry of the instrument is parameterized. Depending on the applied boundary conditions, bending, torsion, or combined bending-torsion loadings are represented. A micromechanical constitutive law is implemented in Abaqus via the UMAT subroutine to describe the thermomechanical behavior of the Cu-based single crystal SMA. Following the thus obtained numerical results, Coltene-Micromega company has manufactured endodontic file prototypes made of Cu-Al-Be single crystal SMA. A specific setup applying the same boundary conditions of torsion-bending loading is used to characterize these SMA file prototypes. The good agreement between experimental and numerical responses, for a combined bending-torsion loading, proves the relevance of the proposed approach and the pertinence of considering Cu-based SMAs for endodontic file applications.

Role of β(FeAl) nanoparticles in abnormal grain growth in the annealing of cast Cu-Al-Mn-Fe shape memory alloys

In this study, the low-cost production of Cu-Al-Mn-Fe shape memory alloy single crystals exceeding 46 mm by abnormal grain growth was realized only through annealing their cast alloys. The results show that the misorientation formed during annealing may be responsible for such abnormal grain growth process. It was confirmed that this misorientation resulted from the dissolution of bcc β(FeAl) nanoparticles during heat treatment at a sufficiently temperature of approximately 1173 K. The rate of migration of the abnormal grain boundary was experimentally measured to be approximately 9.3 × 10-5 m s-1 within 2 min of the commencement of abnormal grain growth. Additionally, the range of composition of the alloys that can lead to abnormal grain growth was determined. When the Cu-13.0Al-6.5Mn-3.2Fe single crystal close to the [100] direction was deformed to a pre-strain of 12%, full shape recovery happened without any residual strain. At that time, the superelastic strain was approximately 9%. Such a superelastic characteristic remained nearly constant over 50 cycles, showing excellent fatigue resistance. The superelastic properties of the present Cu-13.0Al-6.5Mn-3.2Fe single crystal are compared to those of commercial Ni-Ti-based shape memory alloys. Therefore, it can be considered as a new kind of superelastic material having practical applications. The obtained results should be of great significance in the development of Cu-based shape memory alloys. Furthermore, it is expected that a similar microstructure can be designed for the production of more metallic single crystals.

Ambient effects on the output strain of Ni–Mn-Ga single crystal magnetic shape memory alloy

Abstract With proper magneto-mechanical driving forces (e.g., a high-frequency magnetic field plus a mechanical force), Magnetic Shape Memory Alloy (MSMA) can provide a large cyclic deformation (strain oscillation amplitude up to 6%), which makes it a good candidate for high-frequency large-stroke actuators. Moreover, as a kind of shape memory alloys, MSMA’s magneto-mechanical coupling behaviors are very sensitive to temperature, which allows researchers/engineers to modify the strain oscillation amplitude for a wider range of applications by controlling the working temperature. In this paper, we report systematic experiments on the thermal-ambient-dependent oscillation amplitude of the magnetic-field-induced martensite reorientation—utilizing compressed air (with controlled airflow velocity) passing through the surface of the MSMA specimen (Ni–Mn-Ga single crystal) to control the heat transfer between the ambient and the MSMA specimen during the high-frequency magneto-mechanical loading. It is found that the extremely weak or extremely strong ambient heat transfer can only have small strain oscillation amplitude while the maximum strain amplitude can be achieved only at a mild heat-transfer condition (i.e., non-monotonic dependence of the strain amplitude on the ambient heat transfer). It is also demonstrated that the strain amplitude is closely related to the working temperature, satisfying the balance between the heat generation (from the dissipative strain oscillation of the martensite reorientation) and the heat transfer to ambient (due to the temperature difference between the MSMA specimen and the ambient). With such understanding and constraints, three different schemes for controlling the strain amplitude by the thermal method are proposed and tested for their robustness/reliability. These results not only provide some guidelines/principles for designing MSMA actuators with flexible strain amplitude, but also demonstrate the delicate dynamics of the thermo-magneto-mechanical coupling that demand further theoretical/modelling study.

Excellent pseudoelasticity of Al-rich Fe–33Mn–17Al–6Ni–0.15C (at%) shape memory single crystals obtained without an aging conditioning stage

The pseudoelastic behavior of a series of Fe–33Mn–17Al–6Ni–0.15C (at%) shape memory alloy single crystals was analysed with compression tests. The specimens, with different crystallographic orientations, showed almost 100% strain recovery after a full stress-induced martensitic transformation. The size and distribution of precipitates introduced in the alloy processing proved to be clearly suitable for achieving a completely reversible pseudoelastic transformation, without the need of additional aging in this Al-rich alloy. The transformation strain was analysed using the phenomenological theory of martensite transformation. This model roughly describes the pseudoelastic strain under compression as a function of the crystallographic orientation.